Method of flotation of metals



Patented Apr. 28, 1931 UNITED STATES PATENT OFFICE GEORGE G. THOMAS, OF HILLSIDE, ARIZONA, ASSIGNOR TO AMERICAN GYANAMID COMPANY, OF NEW YORK, N. Y., A CORYORATION OF MAINE METHOD OF FLOTA'IION F METALS N0 Drawing.

1igs, tables and the like are em loyed. These gravity methods, although e ective for the recovery of the coarser metallic particles, do not give good recoveries of theinore finely divided metals. Ordinary methods of flotation concentration have'not been effective because such methods, although satisfactory for the concentration of the metals present in the sulfide form, are not satisfactory for the flotation of the free metals themselves.

According to the usual practice in the flotation art, lime is added to the flotation pulp for the purpose of removing from solution such soluble salts as those of iron and copper which frequently cause difliculty in such flotation. To insure complete removal of such salts an excess of lime is added, that is, an amount suiiicient to show a distinct alkalinity when the filtered mill solution is titrated with standard 'acid using phenolphthalein as an indicator. Alkalinities of from 0.01% to 0.10% calculated as free lime (CaO) are usually employed.

I have found that this procedure, although satisfactory for the treatment of ores in which the valuable metals are present as sulfides, is wholly unsuited to the treatment of ores in which the metal occurs in the free state. I have found that lime, both the free base and its salts, is a powerful depressor of flotation for such metallics, and my invention relates to a process whereby this depressing effect is eliminated, either by removing such soluble salts from solution or by so controlling their amount that the depressing effect will be minimized.

According to my invention, the ore to be treated, whether gold, silver or copper, is

Application filed January 26, 1929. Serial No. 335,395.

ground in a ball mill or other device to the fineness required for flotation. I then add to the ore pulp, either before, during or after the grinding, an amount of sodium carbonate sufficient to remove substantially all of the soluble salts of lime. The proper addition is most conveniently measured by observing the pH of the resulting solution. I prefer an alkalinity (pH) of 7.5 to 8.0 although I may even go as high as 9 or as low as 6 Without greatly effecting the results.

Instead of sodium carbonate, I may use a soluble alkali phosphate such as dior trisodium phosphate, a borate, or in fact, any other soluble salt capable of removing from solution the undesired lime salts.

When ores which contain considerable quantities of soluble sulfates of such metals as iron, zinc or copper, are being treated by flotation, it is usually desirable to precipitate such salts from solution before conductin g the flotation operation. This may be done by the addition of sodium carbonate as described above. But since sodium carbonate is a somewhatexpensive reagent, the amounts required may become uneconomical. I have found that lime may be used to remove such. soluble salts provided the lime addition is made in the manner hereinafter described.

According to my invention herein disclosed, the lime is added very carefully until a pH of 7.5 to 8. or possibly slightly higher, is secured, but in any case not to exceed pH 9. By making the lime addition in this way detrimental soluble salts may be removed without employing an excess of lime, and consequently the depressing effect of lime salts upon the flotation of the metallics is avoided. The difference between m procedure of adding lime until a pH in t e neighborhood of pH8 is reached and the old procedure of adding lime until the solution contained an alkalinity titratable against phenolphthalein, is a matter of the very greatest importance and failure to recognize this difference is in large so measure res nsible for the lpoor recovery of metallics w ich has genera 1y attended the flotation of ores containing free metal. Phenolphthalein remains colorless until a pH of close to 9 'is reached and does not become completel pink until H 10. When it is reca led t at the true a alinity re resented b pH 10 is 100 times that of a solution whose a kalinity is H8, itis not surprising thgt such great d i erences are noted 1n the flotability of the metals under such different conditions. So far as I am aware, the uliar depressing action of lime salts upon t e flotation of free metals has never before been observed.

For the determination of the hydrogen ion concentration of such mill solutions I usually employ indicator solutions such as those described in Mansfield Clarks book The Determination of Hydrogen Ions published b Williams and 'Wilkins of Baltimore, M The indicators foundmost useful over the desired range are Brom cresol purple Range 5.4-7 .0 Brom thymol blue Range 6.0-7.6 Phenol red Range 6.6-8.2 Cresol red Range 7.2-8.8 Metacresol purple"; Range 7.4-9.0

The significance of the term pH is also explained in the book.

My complete process, therefore, consists in the preparation of the ground ore pulp in the usual manner, removing soluble sa ts of lime, if such 'be present, b use of soda ash, or adding lime as uired to adjust the pH value, in either case t e amount of alkali employed bein suflicient to impart to the resulting pulp 1 a p of about 8,.or in an case not more than 9 or less than 6. Any 0 the well known reagents such as xanthate, cresylic acid, pine oil or other promoters or frothers are then added and the mixture is subjected to the froth flotationoperation in the known manner. In-

stead of xanthate and pine oil, I may use, and in fact I prefer to use, a mixture of cresylic acid and a diaryldithiophosphate. However, I do not wish to be limited to any particular promoter-frother combination since the essence of m invention is not in the use of such reagents ut in the regulation of the hydrogen-ion concentration of the solution to which these reagents are applied.

As an example of the detrimental efiect of lime salts upon the flotation of metallic gold we show the following results obtained with an ore from the Mount Morgan Gold Min- 00., Ltd. in Australia. This ore con tamed 0.2 oz. gold per ton, part of which was combined mechanically. but not chemically, with the iron-bearing mineral. These tests were carried out in the presence of a constant amount, 0.008 lbs., of sodium cyanide r ton of ore, the purpose of which was to epress 6 the flotation of the pyrite, and the amount of lime addition was varied from 1.25 to 6.5 lbs. per ton of ore. It will be seen that, whereas the recovery of the copper remained almost constant, the recovery of the gold was decreased materially when the lime was added at the rate of 3.2 lbs. or moreper ton of ore, equivalent to a pH of greater t an 9.8.

As an illustration of the detrimental effect of lime salts upon the flotation of metallic copper, we show in the table below the results secured with an ore from the Nevada Consolidated Copper Company, assaying 1.87%

copper and 14.8% iron, and containing some metalhc copper.

m? ConeAesey B "a 6 Tell 5 a Test 2.1m g5 :5 pH

. I! 3 0:0 Cn% rm, 8 Q8 s 1.. an None 1. sea as: sum canes-at an 1.0 12. 20.5 0.41 1a 1a aeuae-m 0.2: so 10. 13.5 0.87 eagle 1am 7.8-8.2

Mixture of cres'yllc acid and dlsubetltuted dith1ophoe phorlc acid. e

n will be seen from this table that the addition of lime had a distinctly depressing effectoupon the flotation of the copper'.

Because of the presence of iron minerals in'the ore in relatively large amounts, and since it is desirable to prevent the entrance into the concentrate of said iron minerals, I generally add to the pulp prior to the flotation .o fiaration sodium cyanide or other cyanides ing in the concentrate the substantial part of the metal resent in the elemental state.

This result .as long been desired and sought for, but to the best of my knowledge had never been obtained prior to my invention.

Another important feature of my invention is the use of an alkaline material to obtain a pH value of less than 9. This is definitely distinguished from the addition of alkaline material in the ordinary flotation of sulfide ores, wherein the alkalinity is much greater than pH 9. It is also distinguished from the flotation of iron minerals in acid circuit, in that my invention contemplates the use of alkaline material added to the pulp, whereas no such material is added in acid flotation. These and other features of my invention distinguish the same from the prior art and contribute to the highly desirable result of obtaining the flotation of metals in the elemental state. My invention is to be construed broadly except as limited by the claims appended hereto.

I claim 1. A method of flotation which comprises providing a pulp of an ore containing metal in the elemental state, precipitating at least the major portion of the soluble salts therein, adjusting the pH value of said pulp to less than 9, adding a frothing agent thereto, and subjecting the pulp to a froth flotation operation to recover said metal.

2. A method of flotation which comprises providing a pulp or an ore containing metal in the elemental state, precipitating at least the major portion of the soluble salts therein, adjusting the pH value of said pulp to 6 to 9, adding a frothing agent thereto, and subjecting the pulp to a froth flotation operation to recover said metal.

3. A method of flotation which comprises providing a pulp or an ore containing metal in the elemental state, precipitating at least the major portion of the soluble salts therein, adjusting the pH value of said pulp to less than 9, adding a frothing agent and a promoting agent thereto, and subjecting the pulp to a froth flotation operation to recover Sal-i metal.

4. A method of flotation which comprises providing a pulp of an ore containing metal in the elemental state, precipitating at least the major portion of the soluble salts therein, adjusting the pH value of said pulp to less than 9, adding a substance capable of depressing the iron mineral in the pulp, adding a frothing agent thereto, and subjecting the pulp to a froth flotation operation to recover said metal.

' 5. A method of flotation which comprises providing a pulp of an ore containing metal in the elemental state, precipitating at least the major portion of the soluble salts therein by the addition of alkaline material at least a part of which is a sodium compound, adjusting the pH value of said pulp to less than 9, adding a frothing agent thereto and subj ecting the pulp to a froth flotation operation to recover said metal.

6. A method of flotation which comprises providing a pulp of an ore containing metal in the elemental state, precipitating at least the major portion of the soluble-salts therein by the addition of alkaline material at least a part of which is a sodium compound, adj usting the pH value of said pulp to 6 to 9, adding a frothing agent thereto and subjecting the pulp to a froth flotation operation to recover said metal.

7. A method of flotation which comprises providing a pulp of an ore containing metal in the elemental state, precipitating at least the major portion of the soluble salts therein by the addition of alkaline material at least a part of which is asodium compound, adjusting the pH value of said pulp to less than 9, adding a frothin agent and a promoting agent thereto an subjecting the pulp to a froth flotation operation to recover said metal.

8. A-method of flotation which comprises providing a pulp of an ore containing metal in the elemental state, precipitating at least the major portion of the soluble salts therein by the addition of alkaline material at least a part of which is a sodium compound, adjusting the pH value of said pul to less than 9, adding a substance capable o depressin the iron mineral in the pulp, adding a frot ing agent thereto and subjecting the pulp to a froth flotation operation to recover said metal.

In testimony whereof, I have hereunto subscribed my name this 15'th dayof December,

GEORGE G. THOMAS. 

